The present invention relates to an optical communication link for sending and receiving an optical signal by using an optical fiber. More particularly, the present invention relates to an optical communication link applicable to in-home communication, communication between electronic devices, and a LAN (Local Area Network) by using a multi-mode optical fiber such as a POF (plastic optical fiber) as a transmission medium.
Normally, in optical fiber transmission, the optical intensity of a light emitting element is set to a value at which with a longest optical fiber having a maximum transmission loss, a photodiode (hereinafter referred to as PD) serving as a light receiving element can detect more than a minimum amount of light which satisfies a predetermined S/N ratio.
In such a method, the light emitting element such as a semiconductor laser (hereinafter referred to as LD) is driven at a considerably high emission intensity in anticipation of a maximum loss of the longest optical fiber.
However, in transmitting the optical signal in different distances by using the LAN, it is necessary to consider the case where the transmission distance is short and a transmission loss in the optical fiber is low. Also, in the case where the optical fiber is used in the in-home LAN, it is necessary to cover a wide range of 1 m to 50 m.
The use of the POF is considered in the In-home LAN. The POF has a comparatively large transmission loss. In the range of 1 m to 50 m, the transmission loss of the POF fluctuates in the range of 0.1 dB to 15 dB, including a variation of a light source. In such a case, in a light-receiving system constructed of a PD and an I-V (current-voltage) conversion amplifier (pre-amplifier), it is necessary to secure a considerably large dynamic range.
To solve the problems, generally, in an optical communication module sending and receiving an optical signal by using an optical fiber as a transmission medium, after an optical output emitted from the optical fiber is measured, the optical output is fed back to the associated light emitting element such as an LED or an LD as a method of compensating fluctuations of photocurrent of the PD owing to different lengths of the optical fibers.
As an example, in Japanese Patent Application Laid-Open No. 10-41896, there is disclosed a method of feeding back the difference between a certain reference level and a level of light emitted from the optical fiber to an APC (Auto Power Control) of a driving circuit of the LD. The schematic construction of an optical communication link adopting the method will be described below with reference to FIG. 19.
In the optical communication scheme shown in FIG. 19, a PD 108 receives a laser beam sent from a counterpart. After an amplifier 109 amplifies an output signal of the PD 108, a peak hold section 110 detects the output signal. A differential amplifier 111 computes the difference between an output signal of the peak hold section 110 and a reference level (Rx. Ref) and outputs the difference to an APC 105 through a selector 112. On the other hand, a part of laser beams outputted from an LD 101 is incident on a PD 104. A signal corresponding to the incident beam is outputted to the APC 105 from the PD 104.
In correspondence to the output signal of the PD 104 and the signal outputted from the differential amplifier 111, the APC 105 supplies an LD actuator 102 with a control signal. The LD actuator 102 amplifies transmission data in correspondence to the control signal outputted thereto from the APC 105 and drives the LD 101. This is the method of solving the above problem.
However, the method of utilizing the feedback has a problem that because the feedback circuit itself is complicated and expensive, the cost of the optical communication link is high. In addition, if an LD is used as a light emitting element, the lower limit of the output is, disadvantageously, restricted by an extinction ratio, and the upper limit of the output is also restricted in consideration of the characteristic, life, and safety of a transmitter.
As described above, in the case where POFs are used for the LAN at home, the transmission loss is comparatively high. In the range of transmission distance of 1-50 m, the transmission loss fluctuates in the range of 0.1 dB (min.) to 15 dB (max.), including fluctuations of light sources. Thus, in the light-receiving system constructed of a PD and an I-V conversion amplifier, it is necessary to secure considerably large dynamic range. Therefore it is impossible to cope with variations in a sending system by merely altering the design of an ordinary optical system.
In particular PDs are requested to have large dynamic range in dependence on variations of light sources, or light emitting elements, to be used in a sending apparatus, variations of excitation NA, and difference in lengths of optical fibers. There may be an extreme case in which optical communication cannot be executed.